In telecommunication systems in which variable-frequency test signals are used to check transmission lines and other circuit elements, the amplitude of the incoming signal voltage can be measured with the aid of a variety of rectifiers whose output may be proportional to the RMS, medium or peak value of the signal voltage. All these rectifiers include smoothing circuits designed to integrate the voltage amplitudes over a succession of cycles. If the frequency swing is wide, the smoothing circuits will have to have time constants which are too large to let the output current of the rectifier follow rapid amplitude changes at the upper end of the frequency range. Thus, in conventional measuring systems in which the test frequency is periodically modulated or wobbled in a band whose lower limit is around 10 Hz, distortions are difficult to avoid at the higher audio frequencies.